Corrosion inhibitor for magnesium cells

ABSTRACT

A CORROSION INHIBITOR FOR MAGNESIUM PRIMARY CELLS COMPRISING AN ALKALI METAL PERMANGANATE SALT IS PRESENTED. IN A PREFERRED EMBODIMENT POTASSIUM PERMANGANATE IN A PERCENTAGE OF FROM 1-5 BY WEIGHT IS INCORPORATED IN THE CATHODE MIX WITH 80-86% MANGANESE DIOXIDE, 10-16% CARBON BLACK, AND 1-3% MAGNESIUM HYDROXIDE, BUFFERING AGENT. WHEN THE CELL DISCHARGES PERMANGANATE IONS FORM A THIN, STABLE FILM AT THE ANODE SURFACE THEREBY PREVENTING FURTHER CORROSION AND EXCESSIVE GASSING.

Sept. 25, 1973 SENA, JR

CORROSION INHIBITOR MAGNESIUM CELLS Filed July 2. 1971 iiii INVENTOR LOUIS SENA JR.

ATTORNEYS United States Patent 3,761,317 CORROSION INHIBITOR FOR MAGNESIUM CELLS Louis Sena, Jr., 183 Mountain Ave., Murray Hill. NJ. 07974 Filed July 2, 1971, Ser. No. 159,437 Int. Cl. H01m 21/00 US. Cl. 136-400 M 4 Claims ABSTRACT OF THE DISCLOSURE A corrosion inhibitor for magnesium primary cells comprising an alkali metal permanganate salt is presented. In a preferred embodiment potassium permanganate in a percentage of from l-5 by weight is incorporated in the cathode mix with 80-86% manganese dioxide, -16% carbon black, and 1-3% magnesium hydroxide, buffering agent. When the cell discharges permanganate ions form a thin, stable film at the anode surface thereby preventing further corrosion and excessive gassing.

This invention relates to primary cells of either the wet or dry type, and particularly to an improved primary cell having a metallic magnesium anode and a carbon cathode depolarized by an admixture of carbon and the depolarizing agent manganese dioxide.

Primary cells having magnesium as the active anodic material, a carbon cathode depolarized with manganese dioxide, and an aqueous solution of [an alkali metal, an alkaline earth metal, or ammonium bromide as the electrolyte have been known for many years and are described in, for example, US. Pats. Nos. 2,547,907 and 2,547,908 to Fry et a1. and 2,621,220 to Kirk et al. The disclosures of these patents concerning the essential features and construction of magnesium cells are hereby incorporated by reference.

Magnesium cells, as described in the aforementioned patents, may be either of the wet or dry type, and theoretically have advantages of high capacity and voltage, and a long shelf life. In addition, magnesium is particularly adapted for use as an anodic material because it is light in weight, readily available, and high in the electro motive series.

Magnesium cells, however, have not found commercial acceptance because the corrodability of the magnesium anode severely limits usefulness of the cell. Corrosion of the metallic magnesium anode may be caused by trace quantities of various metals present therein. The metal is also subject to corrosion attack by the aqueous electrolyte solution to form insoluble, polarizing corrosive products.

Corrosion at the anode is essentially a short-circuited cell formed with a less anodic material that results in solution of the anode, pitting, and ultimately, rupture of the cell. The presence of insoluble corrosion products at the anode also results in concentration polarization by increasing the resistance path between the anode and cathode and by decreasing the rate of diffusion of reactants and products toward or away from the electrode surface. Although an increase in anode polarization through the formation of a protective film or coating on the surface thereof will decrease the corrosion rate, a coating of solid corrosion products is not desirable. In addition to decreasing electrode potential, this coating is not stable or rapidly repairable; it also may function to reduce the area of attack by corrosive agents thereby accelerating corrosion intensity.

The concentrations of substances at the electrode determine the potential of the electrode. As an electrode reaction proceeds, the concentration in the immediate vicinity of the electrode differs from that of the main body of the electrolyte, and as the process continues, ions 3,761,317 Patented Sept. 25, 1973 must continually diffuse away from the electrode or into a film adjacent the electrode. The concentration gradients which set up about the electrode when the cell discharges cause a back electromotive force or polarization. For example, when the magnesium cell is discharged, magnesium ions collect at the magnesium electrode and as their concentration increases, the magnesium metal becomes less anodic.

To minimize magnesium ion concentration an alkaline metal, alkaline earth metal, or ammonium bromide, is provided in the electrolyte to remove the magnesium ion from solution and lower the concentration thereof.

However, a reactive me tal such as magnesium, if permitted, will react to a certain extent with the aqueous electrolyte, even in the absence of oxygen, to form solid corrosion products. These products then collect at the anode surface. Corrosion products at the surface of the electrode tend to form a coating thereover. Although this coating may be expected to reduce the corrosion rate, it may also be weakened by action of the bromide ions in the electrolyte. Weakened areas of the coating will tend to break down, and as previously described, corrosion intensity at the weakened areas will be intensified.

Therefore, if corrosion is permitted, the coating formed will polarize the electrode, and also foster further corrosion and pitting. It is essential, therefore, to provide a substance in the cell that will form a thin protective, corrosion-inhibiting film at the anode surface which will not tend to weaken or break down, and which will be readily repaired as the cell discharges.

In general, oxydizing anions and anions that form insoluble compounds with metals are known to aid in film formation and film repair. As examples, soluble chromates, phosphates, silicates, borates, and hydroxides are known to protect certain metals in aqueous environment.

In the aforementioned patents, and the literature known, the preferred corrosion inhibitors have been soluble chromic acid salts of alkali, akaline earth, and ammonium bases, or water-insoluble chromates such as those of barium, lead, or zinc. In addition to being preferred corrosion inhibitors in magnesium primary cells for many years, chromates have been useful as anticorrosion agents with a wide variety of other metals because of their ability to form a protective film at the surface thereof.

Even though a wide variety of film-forming agents are known, the problem of constructing a commercially feasible magnesium primary cell has not been solved. The chromate compounds described above have not adequately increased cell capacity to the extent that the magnesium cell could find commercial acceptance. The lack of effectiveness could be attributed to an insufiiciently stable film, to the fact that the chromate film is not rapidly repairable, or to the fact that chromate film may overly pacify the electrode. Lack of effectiveness may also stem from a combination of these and other, as yet, unknown factors.

For whatever reason, prior to this invention, an effective corrosion inhibitor for magnesium cells was not known.

It has now been discovered that when a water-soluble, alkali metal permanganate salt, such as potassium permanganate, is incorporated in a magnesium cell, in a concentration effective to inhibit corrosion, the cell capacity is substantially increased during intermittent discharge. By using the permanganate salt of this invention as the corrosion inhibitor, a conventional magnesium cell will have an increased capacity sutlicient to render commercial use feasible.

Although it is not known with certainty, it is believed that the permanganate ions form a thinner, more stable, and more rapidly repairable film at the surface of the magnesium anode during cell discharge and upon removing the load according to one or both of the following reactions:

In a preferred version of this invention a conventional electrolyte is employed. The electrolyte is a 2.5 Normal solution of magnesium bromide in water. It is contemplated, however, that the electrolyte may be an aqueous solution of an alkali metal, alkaline earth metal, or ammonium bromide, or a mixture thereof. In addition, the bromides may be present in concentration of from at least 150-500 grams per liter. The inhibitor of this invention may also be added to the electrolyte in a concentration of from 0.01 to grams per liter, if desired.

The cathode is preferably a carbon rod surrounded by a mix of finely divided particles comprising 1-5% potassium permanganate, 80-86% maganese dioxide, 10- 16% carbon black, and 13% magnesium hydroxide, a buffering agent.

A conventional magnesium anode is employed preferably having a purity of at least 99.5%. Commercial magnesium or magnesium alloys may be utilized, but the presence of trace quantities of other metals will increase the corrodability of the anode. Magnesium alloys, such as AZ-21 and AZ-31 are also preferred because of their ability to limit corrosion caused by small amounts of impurities, such as iron.

The anode may be formed as a plate, sheet, rod, or cup in a conventional manner, and the cathode may be similar to that employed in a standard Leclanch cell, i.e., a carbon rod or plate embedded in a depolarizer admixture of finely divided carbon black and magnesium dioxide. The admixture, however, as noted above, also incldues the permanganate inhibitor of this invention and a buffering agent.

It will be obvious to those skilled in the art that the corrosion inhibitor of this invention may be utilized in magnesium primary cells of either the wet or dry type. This invention is not intended to be limited by the type of electrolyte preferred or the method of forming the cathode or anode.

Accordingly, it is an object of this invention to provide an effective corrosion inhibitor for magnesium primary cells.

It is another object to provide a magnesium primary cell having an anode stabilized by the presence of a permanganate corrosion inhibitor.

It is still another object to provide a magnesium primary cell having a commercially acceptable long shelf life, reduced delay action, and increased cell capacity.

It is a further object of this invention to provide a rapidly stabilized magnesium anode for a primary cell, stabilized by a thin corrosion inhibiting film formed by an alkali metal permanganate salt.

It is yet another object to provide magnesium primary cell having a water-soluble corrosion inhibiting alkali metal permanganate salt incorporated in the cathode mix and the aqueous bromide electrolyte to prevent wasteful corrosion and excessive gassing at the anode surface.

These and other objects will be readily apparent with reference to the following discussion and drawing where- The drawing is a cross-sectional view of a dry cell containing the corrosion inhibitor of this invention.

With reference to the drawing, the cell shown comprises a container 10, which is a cup formed of magnesium metal serving as the anode. The anode has a terminal post 11. Any conventional magnesium metal or magnesium base alloy anode material may beutilized as desired. The cathode 12 is a carbon rod, having a terminal post 14, set in a bobbin 16 of depolarizer mix.

The depolarizer is an admixture of magnesium dioxide,

carbon, the corrosion inhibitor of this invention, and a buffering agent such as magnesium hydroxide. In the preferred embodiment the manganese dioxide depolarizer is present in bobbin 16 in an amount of about -86% by weight, carbon black, the conductive material, is present in from 10-16%, magnesium hydroxide, a buffering agent, is present in from l-3%, and potassium permanganate, the corrosion inhibitor, is present in from 15%. To form the bobbin 16, the admixture is moistened with about 50 ml. of electrolyte per grams of dry mix to obtain a moldable mixture in the conventional manner.

The electrolyte is located in the space 18 between the depolarizer bobbin 16 and the sides of cup 10. The electrolyte is preferably a 2.5 Normal solution of magnesium bromide in water, and may optionally contain from 0.l-l0 grams per liter of the corrosion inhibitor of this invention. It should be noted that the electrolyte may be an alkali metal bromide, an alkaline earth metal bromide, or ammonium bromide, or a mixture thereof with magnesium bromide being preferred.

In compounding a dry cell according to this invention, the anode may take the form of the vessel in which the other elements are contained and the electrolyte may be thickened or jellied as in conventional dry cell practice by the inclusion therein or a thickening agent such as cereal flour or starch or a mixture thereof.

The bobbin 16 preferably is more or less saturated with the aqueous electrolyte and molded around the carbon rod 12 for insertion into the container 10. The bobbin 16 fits inside the container 10 leaving a space 18 between the outside of the bobbin and the inside of the vessel. The space 18 is occupied by the thickened electrolyte. Altenatively, the space 18 may be filled with a porous material such as blotting paper or kraft paper soaked with the electrolyte.

The bobbin 16 is prevented from touching the vessel 10 by insulating bushing 22 at the lower end of the vessel and by Washer 24 and sealing compound 26 at the upper end of the vessel. A washer, not shown, may also be provided at the lower end of the vessel. Binding posts 11 and 14 may be attached to the anode container 10 and cathode 14 as shown, or the cell may be formed without the binding post as in conventional flashlight batteries.

As an example of results achieved with the inhibitor of this invention, primary D cells were constructed as described. The test was constructed by discharging each cell through a 4 ohm resistor for '15 minutes per hour, 10 hours per day, to evaluate the cell capacity. In these tests, the D type cell of this invention took 960 minutes to discharge to 1.1 volt end-voltage, and 1040 minutes to discharge to 0.9 volts end-voltage. This test is comparable to the conventional heavy duty flashlight discharge test, with improved results during intermittent discharge.

In the event the corrosion inhibitor of this invention is desired to be utilized in a wet cell, the cell may be constructed in the conventional manner such as that described in the aforementioned patents to Fry et al. The carbon cathode is surrounded by the cathode mix above described, and the anode is preferably a plate of magnesium. The electrolyte, of course, does not contain a thickening agent.

In summary, this invention contemplates a new corrosion inhibitor useful in any conventional magnesium primary cell to substantially increase the capacity of the said cell. The corrosion inhibitor comprises an alkali metal permanganate salt which may be incorporated in the depolarizer admixture disposed at the cathode of the cell, and may also be dissolved in the electrolyte solution if desired, before discharge of the cell. When the cell discharges the permanganate ions immediately form an extremely effective film on the surface of the anode preventing further corrosion and protecting the anode against pitting, rupture, and excessive gassing during storage and discharge of the cell.

This invention is contemplated for use with any conventional magnesium primary cell, and is not intended to be limited to a cell with electrodes in the shape disclosed or to the construction or method of forming the cathode depolarizer mixture.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A magnesium primary cell comprising a metallic magnesium anode; a solid carbon cathode; a cathode depolarizer mixture including as ingredients finely divided carbon black particles present in from -16% of said mixture, manganese dioxide particles present in from 80- 86% of said mixture, magnesium hydroxide present in from 13% of said mixture, and potassium permanganate present in from 1-5% of said mixture; an aqueous electrolyte solution having a concentration of from 150-500 grams per liter of at least one material selected from the group consisting of alkali metal bromide, alkaline earth metal bromide, and ammonium bromide, said electrolyte References Cited UNITED STATES PATENTS 2,535,742 12/1950 Louzos 136-100 M 3,539,398 11/1970 Ruben 136-100 M 2,547,908 4/1951 Fry et a1. 136-100 M 2,616,940 11/1952 Reid 136-100 M 2,465,443 3/1949 Gide 136-100 3,450,569 6/1969 Dumas et'al 136-100 2,463,316 3/1949 Ruben 136-107 2,536,696 1/1951 Ruben 136-107 2,715,653 8/1955 Reid 136-100 2,952,727 9/1960 Kirk et al 136-120 R ANTHONY SKAPARS, Primary Examiner US. Cl. X.R. 136-107, 138

2 3 3 I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 Dated ptember 25 19 73 Inventor(s) L 4 Sena It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- I V In column 2 line 13, "me tal" should read --metal--.

In column 3, line 35, "incldues" should read --includes-.

In Column 4, line 26, "or" should read --of--.

Signed and sealed this 9th day: of April 19714..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. G. MARSHALL 'DANN Attesting Officer Commissioner of Patents 

